1. Field of the Invention
This invention relates to a monitor that measures blood pressure without using a cuff.
2. Description of Related Art
Blood within a patient's body is characterized by a baseline pressure value, called the diastolic pressure. Diastolic pressure indicates the pressure in an artery when the blood it contains is static. A heartbeat forces a time-dependent volume of blood through the artery, causing the baseline pressure to increase in a pulse-like manner to a value called the systolic pressure. The systolic pressure indicates a maximum pressure in a portion of the artery that contains the flowing volume of blood.
Pressure in the artery periodically increases from the diastolic pressure to the systolic pressure in a pulsatile manner, with each pulse corresponding to an individual heartbeat. Blood pressure then returns to the diastolic pressure when the flowing pulse of blood passes through the artery.
Both invasive and non-invasive devices can measure a patient's systolic and diastolic blood pressure. For example, a non-invasive medical device called a sphygmomanometer measures a patient's blood pressure using an inflatable cuff and a sensor (e.g., a stethoscope) that detects blood flow by listening for sounds called the “Korotkoff” sounds. During a measurement, a medical professional typically places the cuff around the patient's arm and inflates it to a pressure that exceeds the systolic blood pressure. The medical professional then incrementally reduces the pressure while listening for flowing blood with the stethoscope. The pressure value at which blood first begins to flow past the deflating cuff, indicated by a first Korotkoff sound (typically a “beat” or “tap” measured by the stethoscope), is the systolic pressure. The minimum pressure in the cuff that restricts blood flow is the diastolic pressure. The stethoscope monitors this pressure by detecting another Korotkoff sound, in this case a “leveling off” or disappearance in the acoustic magnitude of the periodic beats, indicating that blood flow is no longer restricted.
Low-cost, automated devices measure blood pressure with an inflatable cuff and an automated acoustic or pressure sensor that measures blood flow. The cuffs in these devices are typically fitted to measure blood pressure in a patient's arm, wrist or finger. During a measurement, the cuff is automatically inflated and then incrementally deflated while the automated pressure sensor monitors blood flow. A microcontroller in the automated device then uses this information to calculate blood pressure. Cuff-based blood-pressure measurements only determine the systolic and diastolic blood pressures; they do not measure dynamic, time-dependent blood pressure.
An invasive device for measuring blood pressure, called a tonometer, is inserted into an opening in a patient's skin and features a component that compresses an artery against a portion of bone. A pressure sensor within the tonometer then measures blood pressure in the form of a time-dependent waveform. The waveform features a baseline that indicates the diastolic pressure, and time-dependent pulses, each corresponding to individual heartbeats. The maximum value of each pulse is the systolic pressure. The rising and falling edges of each pulse correspond to pressure values that lie between the systolic and diastolic pressures.